The practice of microlithography in the electronics industry generally involves the irradiation, through a mask, of a substrate coated with photosensitive material called "photoresist". After exposure, there exists a differential solubility in developer solution between those areas that received irradiation and those masked over. Those photoresists that become more soluble in the irradiated areas are designated "positive" photoresists and the photoresists of the invention fall in this category. The differential solubility of the resist areas leads to a relief image in the shape of the original mask pattern after development is complete. This pattern is then the basis for patterning the semiconductor substrate in further processing steps.
Conventional near ultraviolet (NUV) lithography is practiced in the prior art using novolac resins in combination with diazonaphthoquinone sensitizers. The NUV region of the spectrum covers the range of 350-450 nm. Since the wavelength of the exposing radiation is a factor in determining the ultimate resolution of the photoresist image, there is considerable impetus to practice lithography in the DUV region (220-300 nm). By using this shorter wavelength light, the ultimate resolution of the image is improved which in turn allows for device geometries to continue shrinking.
Conventional resists utilizing novolac technology are not useful in the DUV region due to the excessive absorption of these phenolic polymers. In the prior art, M. J. Bowden describes a number of research efforts to produce a deep UV photoresist system in a monograph found in, ACS Symposium Series #266, Materials for Microlithography, L. F. Thompson, Ed., ACS 1984, p. 39. In addition, copolymers of maleimide with styrene and substituted styrenes used as photoresist are described in the co-pending application of Hopf, et al., U.S. Ser. No. 547,815 filed on Nov. 1, 1983.
Accordingly, a need exists for DUV photoresists and more specifically for a class of resins to service as a DUV transparent, aqueous alkaline soluble polymeric vehicle for photosensitive compounds.